Where does phlegm come from?

We posed this question to Dr Suzy Lishman...

Suzy - There is a tube connecting the lungs to the mouth and that's called the trachea. It's the one you can feel - if you just feel the front of your throat just under the Adam's apple, that's the trachea.

That's lined by mucous membranes, the epithelial cells that produce mucus, which is a thick liquid that forms a phlegm. It has several functions; it's partly a lubricant, so it stops the airways from drying out when you breath dry air in and out, and it also helps get rid of bacteria and things that you really don't want to have down in your lungs.

They get stuck in this sticky mucus that lines the airways and then it can be coughed up, and it keep it away from the lungs.

What happens when you get a cold is, in reaction to the extra irritation of your airways, these epithelial cells produce more and more mucus. When you're fit and healthy, the mucus is normally clear and white, but if you get an infection, it can go yellow or even green, as you cough up all the bacteria and the dead cells that your lungs don't want.